Investigate support for webhooks

Webhooks is an area of WireMock that hasn't been touched by WireMocha.

~~Default argument reporting for methods in the webhook() chain~~.
[x] Include webhook serveEventListeners in the JSON-to-Java stub preview.
[x] Code completion of the originalRequest Handlebars variable, and its sub-properties.
- Add a tooltip or type text to this, that it can be used only in webhooks.
[x] Handlebars language injection into webook serveEventListeners.
- Does the serveEventListener name value identify the type of listeners that is used in it, or is it just a simple name?
- URL, method and body are sure to accept Handlebars helpers. Any other property?
[x] Content type header code completion in webhook().withHeader("Content-Type", "<caret>").
- [x] Java DSL
- [x] JSON DSL
[x] Is any method in WebhookDefinition mandatory in order for the webhook to work properly? Other Java side checks?
- [x] withUrl() method
- [x] Is ANY request method supported for webhooks?
- [x] webhook definition used with non-webhook name.
- [x] Either withBody()or withBinaryBody() maybe be specified, but not both.
[x] JSON DSL code analysis
- [x] url property is missing
- [x] Is ANY request method supported for webhooks?
- [x] the headers property must be a map (an object), and not an array or something else.
- [x] Either body or base64Body may be specified, but not both.

Intention action to convert webhook() call chains to WebhookTransformer extension implementation. Is it possible to make such a conversion?
- This could be useful when one wants to provide custom logic for the webhooks.
Inspection: Webhook templates currently do not support system or environment variables.
- This might be obsolete, but have to check. See systemValue Handlebars helper.

picimako / wiremocha